The conventional offset lithographic printing processes employ planographic plates which transfer ink to a blanket roll which then transfers the ink to a substrate thereby forming the printed images. The plates are referred to as planographic since the image and non-image areas are in the same plane. The plates are constructed so that with proper treatment the image areas are rendered hydrophobic and oleophilic and are thereby receptive to inks. The non-image areas are hydrophilic and correspondingly water receptive. In order to maintain the hydrophobicity of the non-image areas, and thereby prevent ink from accumulating on these non-image areas, it is necessary to continuously treat the plate with a water based fountain solution. This is commonly referred to as desensitizing the plate. The water based fountain solution is typically applied with a separate roll prior to inking the plate, or the ink and fountain solution can be applied simultaneously as is typical of the Dahlgren type systems.
A desirable printing ink for use in a lithographic process is one which will have, in addition to other conventional properties, the ability to dry rapidly after being printed on a substrate. Failure of the printed ink to dry rapidly results in several problems such as decreased efficiency of the printing process due to slower press runs. In addition, failure of the ink to dry rapidly results in set off. Set off occurs when printed ink which is not dry adheres to the back of a printed substrate placed on top of it during the stacking of printed substrate as it comes off the presses. This results in a diminution of the appearance of both the printed sides and the non-printed sides of the substrates in a stack.
Due to the nature of the lithographic printing process, lithographic inks are characterized as being oily or greasy since the lithographic inks must be immiscible with water. Lithographic inks typically contain drying oils and drying oil modified alkyds as vehicles. A vehicle is the fluid component of a printing ink which serves as the dispersing and carrying medium for the pigment particles. The vehicle is responsible for the rheological properties of the inks, and the vehicle binds pigment to a printed substrate. The drying properties of lithographic inks are attributable to the chemical characteristics and
Drying oils may be derived from mineral oils, vegetable oils, animal oils or synthetic oils. The most commonly used drying oils are vegetable oils such as linseed oil, chinawood oil, oiticica oil, perilla oil, soya bean oil, etc. The drying oils are characterized by their ability to absorb atmospheric oxygen and form a film. Drying oils generally contain large quantities of glycerides of unsaturated fatty acids such as linoleic acid and linolenic acid, etc.
Lithographic printing inks dry by the oxidation of the drying oils or drying oil modified alkyds contained in these inks. Oxidation of these drying oils is thought to result in a polymeric network thereby producing a dry printed ink. Normally, the unassisted drying process takes days. It is known in the art to include substances in the formulation of these inks to speed up the drying or "setting" process. These substances, conventionally known as driers, increase the rate of transformation of the ink from the liquid phase to the solid phase. Driers, which are actually catalysts, speed up the oxidation rate so that the drying process is typically complete within several hours. Conventional driers used in lithographic inks are typically the metallic soaps of organic acids and inorganic salts. Metallic driers are usually classified as liquid driers or paste driers. The liquid driers comprise oil soluble soaps, while the paste driers comprise dispersions of inorganic salts in oils. The liquid driers are prepared by the reaction of metal oxides or metal salts with saponified organic acids. Typical of the metal driers are cobalt, lead, manganese, cerium, zirconium, lithium, calcium, zinc and iron. The paste driers are typically prepared by grinding lead and manganese organic salts, such as lead acetate and manganese borate, into linseed oil varnishes. Inks containing paste driers dry more slowly than inks containing liquid driers. Different metallic driers have different effects on the drying characteristics of a lithographic ink. For example, cobalt driers catalyze surface drying of lithographic inks, while lead driers catalyze through drying of lithographic inks.
The amount of drier which can be included in a printing ink is limited to an optimum amount. Above this optimum, several problems are encountered resulting in unacceptable ink characteristics such as decreased shelf storage life, poor printing characteristics due to premature drying, and continually decreasing aesthetic appearance of the printed ink over time due to continuing oxidation after drying caused by the high concentration of drier.
As stated above, in order to prevent these inks from accumulating in the non-image areas, it is necessary to continually treat the plates with fountain solution. Although plain tap water can be used as a fountain solution, it is known in the art to include additives to enhance or improve the printing characteristics. It is known to include low boiling point alcohols such as isopropanol and ethanol, and various surfactants to reduce the surface tension of the solution thereby producing better wetting of the plate. It is also known to add acids and/or buffers to achieve and maintain a certain pH range. Glycerine is typically added to fountain solutions to act as a humectant and lubricant. Acid containing fountain solutions typically contain sufficient quantities of gum arabic to protect the plate from excessive etching. Bactericides and algicides are added to control microbiogical growth. Typically these multi-component fountain solutions are sold by the manufacturer as a concentrate. The fountain solutions are prepared at the printing site by diluting the fountain solution concentrates with water.
The fountain solution can be very important during a press run with regard to improving the lithographic printing characteristics by eliminating or reducing process problems such as greasing or scumming, tinting, stripping, poor ink transfer and piling. An improperly formulated fountain solution will result in an increase in the drying time of the printed ink.
Driers have been included in fountain solutions to improve the drying time of the printed ink. For example, cobalt acetate has been included in the formulation of fountain solutions. Because of the nature of the lithographic printing process, a certain amount of fountain solution will be emulsified in the printed ink thereby resulting in a transfer of the cobalt acetate, a drier, in the fountain solution to the printed ink. The resultant increased concentration of drier in the printed ink produces a decrease in the drying time of the ink. There are several problems associated with the use of driers such as cobalt acetate in fountain solutions, including erratic drying time of the printed ink over the press run. This is caused by a typically increased concentration of the drier in the fountain solution over time during a press run resulting in an increased concentration of the drier in the ink and a consequent variable reduction in drying time. In addition to the drying time of the printed ink being erratic, the increased concentration of drier causes premature drying of ink, typically resulting in a build-up of ink on the press rollers which will ultimately result in a decreased quality of the printed image.
Accordingly, what is needed in the art is a means for reducing the drying time of printing inks without increasing the concentration of driers in the inks.